1. Field of the Invention
The present invention relates to a CMOS bias circuit having a starter circuits.
2. Background Art
A conventional semiconductor integrated circuit includes a starter circuits part 3, a constant current circuit part 4, and an output takeout circuit part 5 (see, for example, Japanese Patent Laid-Open No. 2003-110032 (FIG. 5)).
The constant current circuit part 4 has two stable operating points. At one of the two operating points, both currents I1 and I2 are zero. The other of the two operating points is a desired operating point, which is an operating point depending upon size ratios W/L and differences between threshold voltages of transistors M1 to M4, and a resistance value of a resistor R1. If the currents I1 and I2 are provided with suitable starter currents at the desired operating point, and the constant current circuit part 4 makes a transition to the desired operating point or the constant current circuit part 4 satisfies a condition for transition to the desired operating point, then it is necessary to remove the starter current and thereby prevent the constant current circuit part 4 from deviating from the desired operation point.
A resistor R2 and transistors M5, M6, M8 and M9 in the starter circuits part 3 supplies a starter current I4 to the constant current circuit part 4, and the constant current circuit part 4 supplies a gate bias voltage to a transistor M7 in the starter circuits part 3.
When the constant current circuit part 4 is conducting the desired operation, the transistor M7 supplied with the gate bias voltage supplies a current I5 to a connection node between the transistor M8 and the transistor M9 to stop the starter current. If the current I5 exceeds the current supply capability of the transistor M9, then the potential at the connection node between the transistor M8 and the transistor M9 rises, and the transistor M8 is biased to turn off, the current I4 being interrupted.
For example, when using the semiconductor integrated circuit with a lower power supply voltage, there is a problem that the current of the transistor M8 is not interrupted.
The transistor M8 is a current controlled current switch transistor for interrupting the current output from the transistor M9. If the potential difference between the power supply and the ground is small (the power supply voltage is low), the potential at the drain of the transistor M8 might become lower than the potential at the gate of the transistor M8.
If the potential difference between the gate potential and the drain potential (VCP potential at the output of the starter circuits part 3) becomes higher than a threshold voltage Vth of the transistor M8, then a current which is opposite in direction to the starter current I4 flows through the transistor M8 and exerts an influence upon the VCP potential.
For example, the opposite direction current of the transistor M8 changes the currents I1 and I2 of the constant current circuit part 4 to values which are different from those intended, and the deviations are influenced by the power supply voltage.
In this way, when the power supply voltage is low, the conventional art has a problem that the current which is opposite in direction to the starter current flows through the current controlled current switch transistor and influence of the variation of the power supply voltage is exerted upon a started circuit part (the constant current circuit part 4 and an output takeout circuit part 5) via the starter circuits part 3.